Methods of forming flash devices with shared word lines

ABSTRACT

Word lines of a NAND flash memory array are formed by concentric, rectangular shaped, closed loops that have a width of approximately half the minimum feature size of the patterning process used. The resulting circuits have word lines linked together so that peripheral circuits are shared. Separate erase blocks are established by shield plates.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to application Ser. No.: ______, entitledFlash Devices With Shared Word Lines to Masaaki Higashitani, filedconcurrently and is hereby incorporated by this reference in itsentirety.

BACKGROUND

This invention relates to flash memory arrays and in particular to thestructures of flash memory arrays and methods of forming them.

All patents, patent applications, publications and other referencescited in the present application are hereby incorporated by reference intheir entirety.

There are many commercially successful nonvolatile memory products beingused today, particularly in the form of small form factor cards, whichuse an array of flash EEPROM (Electrically Erasable and ProgrammableRead Only Memory) cells. Such cards may be interfaced with a host, forexample, by removably inserting a card into a card slot in a host. Someof the commercially available cards are CompactFlashTm (CF) cards,MultiMedia cards (MMC), Secure Digital (SD) cards, Smart Media cards,personnel tags (P-Tag) and Memory Stick cards. Hosts include personalcomputers, notebook computers, personal digital assistants (PDAs),various data communication devices, digital cameras, cellulartelephones, portable audio players, automobile sound systems, andsimilar types of equipment. In an alternative arrangement to theseparate card and host described above, in some examples a memory systemis permanently connected to a host providing an embedded memory that isdedicated to the host.

An example of a prior art memory system 100 is generally illustrated inthe block diagram of FIG. 1. A large number of individually addressablememory cells are arranged in a regular array 110 of rows and columns,although other physical arrangements of cells are certainly possible.Bit lines, designated herein to extend along columns of the array 110,are electrically connected with a bit line decoder and driver circuit130 through lines 150. Word lines, which are designated in thisdescription to extend along rows of the array 110, are electricallyconnected through lines 170 to a word line decoder and driver circuit190. Each of the decoders 130 and 190 receives memory cell addressesover a bus 160 from a memory controller 180. The decoder and drivingcircuits are also connected to the controller 180 over respectivecontrol and status signal lines 135 and 195.

The controller 180 is connectable through lines 140 to a host device(not shown). The host may be a personal computer, notebook computer,digital camera, audio player, various other hand held electronicdevices, and the like. The memory system 100 of FIG. 1 will commonly beimplemented in a card according to one of several existing physical andelectrical standards, such as one from the PCMCIA, the CompactFlash™Association, the MMC™ Association, and others. When in a card format,the lines 140 terminate in a connector on the card that interfaces witha complementary connector of the host device. The electrical interfaceof many cards follows the ATA standard, wherein the memory systemappears to the host as if it was a magnetic disk drive. Other memorycard interface standards also exist. In some systems, a memory card maynot have a controller and the functions of the controller may be carriedout by the host. As an alternative to the card format, a memory systemof the type shown in FIG. 1 may be permanently embedded in the hostdevice.

The decoder and driver circuits 130 and 190 generate appropriatevoltages in their respective lines of the array 110, as addressed overthe bus 160, according to control signals in respective control andstatus lines 135 and 195, to execute programming, reading and erasingfunctions. Any status signals, including voltage levels and other arrayparameters, are provided by the array 110 to the controller 180 over thesame control and status lines 135 and 195. A plurality of senseamplifiers within the circuit 130 receive current or voltage levels thatare indicative of the states of addressed memory cells within the array110, and provides the controller 180 with information about those statesover lines 145 during a read operation. A large number of senseamplifiers are usually used in order to be able to read the states of alarge number of memory cells in parallel. During reading and programoperations, one row of cells is typically addressed at a time throughthe circuits 190 for accessing a number of cells in the addressed rowthat are selected by the circuit 130. During an erase operation, allcells in each of many rows are typically addressed together as a blockfor simultaneous erasure.

Two general memory cell array architectures have found commercialapplication, NOR and NAND. In a typical NOR array, memory cells areconnected between adjacent bit line source and drain diffusions thatextend in a column direction with control gates connected to word linesextending along rows of cells. A memory cell includes at least onestorage element positioned over at least a portion of the cell channelregion between the source and drain. A programmed level of charge on thestorage elements thus controls an operating characteristic of the cells,which can then be read by applying appropriate voltages to the addressedmemory cells. Examples of such cells, their uses in memory systems andmethods of manufacturing them are given in the following U.S. Pat. Nos.5,070,032; 5,095,344; 5,313,421; 5,315,541; 5,343,063; 5,661,053 and6,222,762. These patents, along with all other patents, patentapplications and other publications referred to in this application arehereby incorporated by reference in their entirety for all purposes.

In a NAND array series strings of more than two memory cells, such as 16or 32, are connected along with one or more select transistors betweenindividual bit lines and a reference potential to form columns of cells.Word lines extend across cells within a large number of these columns.An individual cell within a column is read and verified duringprogramming by causing the remaining cells in the string to be turned onhard so that the current flowing through a string is dependent upon thelevel of charge stored in the addressed cell. An example of a NANDarchitecture array and its operation as part of a memory system is foundin the following U.S. Pat. Nos. 5,570,315; 5,774,397; 6,046,935 and6,522,580. NAND memory devices have been found to be particularlysuitable for mass storage applications such as those using removablememory cards.

FIG. 2A shows a portion of EEPROM cell array 110 of FIG. 1 having a NANDstructure. Only a small portion of the repetitive structure is shown.NAND strings of memory cells are formed extending in the Y-direction.NAND strings include implanted source/drain regions that connectindividual memory cells. A memory cell includes a floating gateoverlying a channel region in the substrate. A series of word lines,WL0-WL3 extend across the memory array in the X-direction and overliefloating gates of memory cells of different strings. In addition, selectgate lines (SSL, DSL) extend in the X-direction at either end of theNAND strings and overlie portions of the substrate to form select gatesof select transistors that control the connection of NAND strings tomemory control circuits. At one end of the NAND strings, a common sourceline (not shown) connects to each of the NAND strings. At the other endof the NAND strings, connections are made to bit lines (not shown). In atypical NAND memory array, NAND strings that share word lines and selectlines form a block in the memory array that is erased as a unit. Atypical string may include many memory cells, with 8, 16, 32 or morememory cells in a string being common. Thus, a typical block may have 32or more word lines extending across the NAND strings of the block. Ablock may have thousands of strings that are spaced apart in theX-direction. FIG. 2B shows a circuit diagram for the physical structureof FIG. 2A. FIG. 2B includes the common source line connecting the NANDstrings at one end. NAND strings are shown extending between bit lineconnections and common source connections with select transistorscontrolling these connections.

FIG. 2C shows a cross sectional view of a NAND string of FIG. 2A(indicated by A-A in FIG. 2A). FIG. 2C more clearly shows the structureof individual memory cells having a floating gate (FG) formed from afirst polysilicon layer (P1) and a control gate (CG) formed from asecond polysilicon layer (P2). The control gate is formed by a portionof a word line that overlies a floating gate. In between a floating gateand a control gate is a dielectric layer 19. In addition, FIG. 2C showsimplanted source/drain regions connecting adjacent cells in the NANDstring. A gate dielectric layer is shown insulating floating gates fromthe substrate. Metal bit line contact and source contact are shown ateither end of the NAND string. A source select transistor and a drainselect transistor are shown having portions of both first polysiliconlayer P1 and second polysilicon layer P2. For select transistors, thesetwo layers are connected together so that no floating gate is formed.Alternatively, a single polysilicon layer may be used to form selectgates.

The charge storage elements of current flash EEPROM arrays, as discussedin the foregoing referenced patents, are most commonly electricallyconductive floating gates, typically formed from conductively dopedpolysilicon material. An alternate type of memory cell useful in flashEEPROM systems utilizes a non-conductive dielectric material in place ofthe conductive floating gate to store charge in a non-volatile manner. Atriple layer dielectric formed of silicon dioxide, silicon nitride andsilicon oxide (ONO) is sandwiched between a conductive control gate anda surface of a semi-conductive substrate above the memory cell channel.The cell is programmed by injecting electrons from the cell channel intothe nitride, where they are trapped and stored in a limited region, anderased by injecting hot holes into the nitride. Several specific cellstructures and arrays employing dielectric storage elements and aredescribed in U.S. Pat. No. 6,925,007. Thus, while examples given in thepresent application may refer to floating gates, other charge storagestructures may also be used. The present application is not limited to aparticular charge storage structure.

As in most integrated circuit applications, the pressure to shrink thesilicon substrate area required to implement some integrated circuitfunction also exists with flash EEPROM systems. It is continuallydesired to increase the amount of digital data that can be stored in agiven area of a silicon substrate, in order to increase the storagecapacity of a given size memory card and other types of packages, or toboth increase capacity and decrease size. One way to increase thestorage density of data is to store more than one bit of data per memorycell. This is accomplished by dividing a window of a floating gatecharge level voltage range into more than two states. The use of foursuch states allows each cell to store two bits of data, eight statesstores three bits of data per cell, and so on. A multiple state flashEEPROM structure and operation is described in U.S. Pat. Nos. 5,043,940and 5,172,338, which patents are incorporated herein by this reference.

Increased data density can also be achieved by reducing the physicalsize of the memory cells and/or the overall array. Shrinking the size ofintegrated circuits is commonly performed for all types of circuits asprocessing techniques improve over time to permit implementing smallerfeature sizes. But there are usually limits of how far a given circuitlayout can be shrunk in this manner, since there is often at least onefeature that is limited as to how much it can be shrunk. When thishappens, designers will turn to a new or different layout orarchitecture of the circuit being implemented in order to reduce theamount of silicon area required to perform its functions. The shrinkingof the above-described flash EEPROM integrated circuit systems can reachsuch limits.

One way to form small cells is to use a self-aligned Shallow TrenchIsolation (STI) technique. This uses STI structures to isolate adjacentstrings of floating gate cells such as those of NAND type memory arrays.According to this technique, a gate dielectric (tunnel dielectric) layerand floating gate polysilicon layer are formed first. Next, STIstructures are formed by etching the gate dielectric and floating gatepolysilicon layers and the underlying substrate to form trenches. Thesetrenches are then filled with a suitable material (such as oxide) toform STI structures. The portions of the gate dielectric and floatinggate polysilicon layers between STI structures are defined by the STIstructures and are therefore considered to be self-aligned to the STIstructures. Typically, the STI structures have a width that is equal tothe minimum feature size that can be produced with the processingtechnology used. STI structures are also generally spaced apart by theminimum feature size. Thus, the portions of the gate dielectric andfloating gate polysilicon layers between STI regions may also have awidth that is equal to the minimum feature size. The strips of floatinggate polysilicon are further formed into individual floating gates inlater steps.

Another way to form small cells is to reduce the size of the features.However, lithographic processes used to establish the dimensions ofdevices are generally limited by some minimum feature size. Memory cellsare generally designed to have dimensions that are equal to this minimumfeature size (F). Thus, in FIG. 2A, the width of NAND strings and theseparation between adjacent NAND strings is approximately F. Also, thewidth of the word lines and separation between adjacent word lines isapproximately F. In one technique, sidewall spacers are grown that arenarrower than F and used to form word lines that are narrower than F. Anexample of such a technique is described in U.S. Pat. No. 6,888,755.

While memory cells within a memory array may be scaled down in sizeusing various techniques (including providing features that are smallerthan the minimum feature size), peripheral circuits may not always be soeasily scaled. Peripheral circuits include various circuits that are onthe same substrate as a memory array and are used to manage the memoryarray. Examples of peripheral devices include word line decoder anddriver circuits and bit line decoder and driver circuits. Peripheralcircuits may have to withstand relatively large voltages so that theyrequire relatively thick dielectric layers and relatively large devicesizes. Because such peripheral circuits are not generally scaled down insize in proportion to the memory array, these peripheral circuits cometo occupy an undesirably large area on a substrate.

SUMMARY

A memory array is formed using sidewall spacers to pattern a maskinglayer, sidewall spacers having a width that is approximately half theminimum feature size of the lithographic process used. The masking layerpattern is then used to form word lines that overlie and are selfaligned to floating gates. Sidewalls are formed as rectangular shaped,closed loops, and so word lines are also formed as portions ofrectangular shaped, closed loops that connect word lines of adjacentblocks (or in some cases, two word lines of the same block). Two wordlines connected in this manner share word line decoder and drivercircuits. The number of decoder and driver circuits may be reduced byhalf as a result of sharing by two word lines, thus providing asignificant space saving.

A process using sidewalls to form narrow word lines also forms selectlines using sidewalls to define the extent of select lines so thatselect lines are not separately aligned to the memory array but aredefined by the same process steps that define word lines. Photoresistportions are used in formation of select lines, but the locations ofedges of select lines are not determined by photoresist. Instead,locations of edges are determined by sidewalls of the same masking layerwhose sidewalls establish word line locations. While photoresistportions are aligned to the masking layer that defines word lines, thealignment is not critical and a certain amount of misalignment may betolerated because it does not affect the locations of features formed.

Forming contacts to narrow word lines (having a thickness less than theminimum feature size) may be difficult. Contact pads may be providedthat have larger dimensions than the word line. Contact pads are formedby the same steps used to form select lines. Thus, the locations ofcontact pads are determined by sidewalls of the same masking layer usedto establish locations of word lines and select lines. Subsequently,photoresist portions are added in a step that is tolerant ofmisalignment.

In some examples, all NAND strings that have word lines connectedtogether are erased together and thus form a single block. This resultsin a block that is similar to two conventional block-like unitsconnected together by word lines. Alternatively, a more conventionalblock arrangement is achieved by providing shield plates for each block,thus making the blocks separately erasable. Shield plates allowdifferent voltages to be coupled to floating gates of different blocks,even though the word lines of the different blocks are connectedtogether and therefore have the same voltage. Thus, a sufficient voltagedifference between floating gates and the substrate can be establishedfor one block to allow erasing, while an adjacent block that shares thesame word lines has a lower voltage difference between floating gatesand the substrate and therefore does not undergo erasing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a memory system of the prior art.

FIG. 2A shows a NAND flash memory array of the prior art.

FIG. 2B shows a circuit diagram for the prior art NAND flash memoryarray of FIG. 2A.

FIG. 2C shows a cross section of a prior art NAND string of FIG. 2A.

FIG. 3 shows a cross section of a NAND flash memory array according toan embodiment of the present invention at an intermediate stage offabrication.

FIG. 4 shows a cross section of the NAND flash memory array of FIG. 3along a direction perpendicular to the cross section of FIG. 3 withphotoresist portions visible.

FIG. 5 shows the structure of FIG. 4 in the same view after photoresistslimming to reduce the width of photoresist portions.

FIG. 6 shows the structure of FIG. 5 after slimmed photoresist portionsare used as an etch mask to pattern an underlying Silicon Nitride layer.

FIG. 7 shows the structure of FIG. 6 after deposition of a Silicondioxide layer that overlies Silicon Nitride portions and exposedpolysilicon.

FIG. 8 shows the structure of FIG. 7 after deposition of photoresistportions to cover areas of the Silicon dioxide layer.

FIG. 9 shows the structure of FIG. 8 after etching of the Silicondioxide layer to remove portions that are not adjacent to sidewalls ofSilicon Nitride portions or covered by photoresist and subsequentremoval of photoresist portions.

FIG. 10 shows the structure of FIG. 9 after removal of Silicon Nitrideportions leaving Silicon dioxide portions on polysilicon.

FIG. 11 shows the structure of FIG. 10 after using Silicon dioxideportions as an etch mask to pattern underlying polysilicon layers toform floating gates, select gates and word lines.

FIG. 12 shows the structure of FIGS. 3 and 4 from above with markingsB-B and C-C to show the views of FIGS. 3 and 4 respectively.

FIG. 13 shows the structure of FIG. 12 after photoresist slimming isperformed to reduce the width of photoresist portions. FIG. 13 shows thesame stage of fabrication as FIG. 5 from a different perspective.

FIG. 14 shows the structure of FIG. 13 after patterning of underlyingSilicon Nitride using slimmed photoresist portions as an etch mask. FIG.14 shows the same stage of fabrication as FIG. 6 from a differentperspective.

FIG. 15 shows the structure of FIG. 14 after deposition of a Silicondioxide layer and formation of photoresist portions to cover parts ofthe Silicon dioxide layer. FIG. 15 shows the same stage of fabricationas FIG. 8 from a different perspective.

FIG. 16 shows the structure of FIG. 15 after etching of the Silicondioxide layer, removal of photoresist portions, removal of SiliconNitride portions and etching of underlying polysilicon layers usingSilicon dioxide portions as an etch mask. FIG. 16 shows the same stageof fabrication as FIG. 11, with additional source and drain contacts.

FIG. 17 shows an equivalent circuit diagram for the structure of FIG. 16with bit lines that are not shown in FIG. 17.

FIG. 18 shows an alternative embodiment at an intermediate stage offabrication including photoresist portions with openings to form contactpads.

FIG. 19 shows the structure of FIG. 18 after photoresist slimming andetching of Silicon Nitride using slimmed photoresist portions as an etchmask, deposition of a Silicon dioxide layer and subsequent formation ofphotoresist portions to cover parts of the Silicon dioxide layer.

FIG. 20 shows the structure of FIG. 19 after etching of the Silicondioxide layer, removal of photoresist portions, removal of SiliconNitride portions, etching of underlying polysilicon layers to form wordlines and floating gates and formation of word line contacts to contactpads.

FIG. 21 shows a cross section of a shield plate formed over a portion ofa NAND array according to an embodiment of the present invention.

FIG. 22 shows the portion of a NAND array with shield plate of FIG. 21in plan view and a second shield plate extending over a second portionof the NAND array.

FIG. 23 shows a cross section of an alternative shield plate formed overa portion of a NAND array according to an embodiment of the presentinvention.

FIG. 24 shows the portion of the NAND array with shield plate of FIG. 23in plan view and a second shield plate extending over a second portionof the NAND array.

FIG. 25 shows a flowchart of a process for fabrication of a NAND flashmemory array according to an embodiment of the present invention.

FIG. 26 shows electrical connections to elements of a NAND arrayaccording to an embodiment of the present invention.

FIG. 27A shows voltage values applied to elements of the NAND array ofFIG. 26 during a read operation.

FIG. 27B shows voltage values applied to elements of the NAND array ofFIG. 26 during a program operation.

FIG. 27C shows voltage values applied to elements of the NAND array ofFIG. 26 during an erase operation.

FIG. 28 shows electrical connections to elements of a NAND array thathas a shield plate according to an embodiment of the present invention.

FIG. 29A shows voltage values applied to elements of the NAND array ofFIG. 28 during a read operation.

FIG. 29B shows voltage values applied to elements of the NAND array ofFIG. 28 during a program operation.

FIG. 29C shows voltage values applied to elements of the NAND array ofFIG. 28 during an erase operation.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 3 shows a cross section of a NAND array according to an embodimentof the present invention at an intermediate stage of fabrication. Theformation of the NAND array up to this point may follow a conventionaltechnique where a first dielectric layer 301 (gate oxide layer) isformed over a substrate 303 and subsequently a first polysilicon layer305 is formed over first dielectric layer 301. First polysilicon layer305 is doped so that it is electrically conductive. STI structures 307a-d are formed by patterning substrate 303 and etching trenches throughfirst polysilicon layer 305 and through first dielectric layer 301. Thetrenches also extend into substrate 303. The trenches are filled withSTI material (a suitable dielectric material such as Silicon dioxide) toprovide electrical insulation between devices. Thus, strips of STImaterial form STI structures 307 a-d that extend across substrate 303(in a direction perpendicular to the cross section of FIG. 3) separatedby strips 305 a-c of first polysilicon layer 305. Both STI structures307 a-d and strips 305 a-c of first polysilicon have a width that is theminimum feature size (F) of the process used for patterning.Subsequently, a second polysilicon layer 309 is deposited that overliesboth STI structures 307 a-d and strips 305 a-c of first polysiliconmaterial. Second polysilicon layer 309 is also doped and electricallyconductive. Second polysilicon layer 309 is separated from strips 305a-c of first polysilicon by a second dielectric layer 311. Subsequently,a masking layer 313 is formed over second polysilicon layer 309. In thiscase, masking layer 313 is formed of a dielectric, Silicon Nitride(SiN), though other suitable masking materials may also be used.

FIG. 4 shows a cross section of the NAND array of FIG. 3 along adirection that is at right angles to the cross section of FIG. 3. Thus,FIG. 4 shows a single strip 305 a of first polysilicon material in crosssection with second polysilicon layer 309 overlying strip 305 a. FIG. 4also shows portions 415 a-e of photoresist overlying masking layer 313.Portions of photoresist 415 a-e are formed by applying a blanket layerof photoresist and then patterning the photoresist using a lithographicprocess. Portions of photoresist 415 a-e may be formed as strips havinga width that is equal to the minimum feature size (F) of thelithographic process used. Portions of photoresist 415 a-e may also bespaced apart by a distance that is equal to F. Other dimensions greaterthan F may also be used. While the present process uses photoresist thatis patterned by being exposed to light, other patterning processes mayalso be used including e-beam lithography.

FIG. 5 shows the NAND array of FIG. 4, along the same cross section,after a resist slimming step is performed. Resist slimming involvessubjecting portions of photoresist 415 a-c to etching to remove at leastsome photoresist and so make portions of photoresist 415 a-e narrower. Aconventional etch may be used for this step, such as a dry etch. In theexample shown, portions of photoresist 415 a-e are narrowed from aninitial width equal to the minimum feature size (F) to about half theinitial width (F/2). The distance between portions of photoresist 415a-e increases accordingly from an initial distance (F) to one and a halftimes the initial distance (3F/2).

Subsequent to resist slimming, the slimmed portions of photoresist areused to pattern the underlying Silicon Nitride masking layer 313. Anetch is performed so that unexposed portions of masking layer 313 areremoved, while those portions of masking layer 313 that are covered byportions of photoresist 415 a-e are not removed. Portions of photoresist415 a-e are then removed. FIG. 6 shows the resulting structure along thesame cross section as FIG. 5. The etch stops when second polysiliconlayer 309 is reached so that second polysilicon layer 309 is notaffected by this step. This patterning step transfers the pattern of theportions of photoresist 415 a-e to masking layer 313 so that maskingportions 313 a-e are formed having a width of F/2 that are separated by3F/2.

FIG. 7 shows the structure of FIG. 6, along the same cross section,after formation of a third dielectric layer 717 that overlies maskingportions 313 a-e and the exposed areas of the second polysilicon layer.In this example third dielectric layer 717 is formed of Silicon dioxide(SiO2 or “oxide”). Third dielectric layer 717 is formed as a blanketlayer by a conventional process such as Chemical Vapor Deposition (CVD).Third dielectric layer 717 is generally a thicker dielectric layer thanfirst dielectric layer 301 and second dielectric layer 311. Thirddielectric layer. 717 extends along second polysilicon layer 309 whereit is exposed and extends along the top surfaces and sidewalls ofmasking portions 313 a-e.

FIG. 8 shows the structure of FIG. 7, along the same cross section,after formation of photoresist portions 819 a, 819 b overlying portionsof third dielectric layer 717. Photoresist portions 819 a, 819 b may beformed by covering the structure with photoresist, then patterning thephotoresist using a lithographic process by removing unwanted portionsof photoresist. Photoresist portions 819 a, 819 b extend over portionsof third dielectric layer 717 that directly overlie second polysiliconlayer 309. Subsequent to formation of photoresist portions 819 a, 819 b,an etch is carried out to remove certain exposed portions of thirddielectric layer 717.

FIG. 9 shows the structure of FIG. 8, along the same cross section,after an etch step is carried out. The etch step may use anisotropicetching such as Reactive Ion Etching (RIE) so that third dielectriclayer 717 is etched through in some places but portions 717 a-f of thirddielectric layer 717 remain along sidewalls of masking portions 313 a-ebecause of the vertical thickness of third dielectric layer 717 in theselocations. Remaining portions 717 a-f of third dielectric layer 717include portions 717 b-e referred to as sidewall spacers because theyare formed along sidewalls of masking portions 313 b-d. The dimensionsof sidewall spacers 717 b-e are determined by the thickness of thirddielectric layer 717 and by the nature of the anisotropic etch used. Inthis case, sidewall spacers 717 b-e have a width of approximately halfthe minimum feature size (F/2), leaving gaps between sidewall spacers717 b-e that are also approximately half the minimum feature size. Afterthe etch is completed, a photoresist strip step is also performed toremove photoresist portions 819 a, 819 b. This leaves wide dielectricportion 717 a that extends between masking portions 313 a and 313 b andwide dielectric portion 717 f that extends between masking portions 313d and 313 e. The dimensions of wide dielectric portions 717 a, 717 f aredetermined by the locations of masking portions 313 a, 313 b, 313 d and313 e, not by the dimensions of photoresist portions 819 a, 819 b. Thus,wide dielectric portions 717 a, 717 f, which subsequently establish thelocations of select gate lines, are aligned with sidewall spacers 717b-e, which subsequently establish the locations of word lines, and donot require separate alignment, unlike many prior art schemes. Theprecise positioning of photoresist portions 819 a, 819 b is not criticalto positioning of wide dielectric portions 717 a, 717 f. Photoresistportions 819 a, 819 b should extend from close to one sidewall to closeto an adjacent sidewall but precise alignment is not required. Edges ofphotoresist portions 819 a, 819 b do not have to coincide with locationsof sidewalls because the thicker dielectric layer in these areas ensuresthat the dielectric will not be etched through. In the present example,wide dielectric portions 717 a, 717 f have a width of approximately fourtimes the minimum feature size (4F). FIG. 9 shows a masking portion 313d having a width of X. In the present example, X is approximately F/2,though in other examples X may be greater than F/2. The distance X laterestablishes a distance between a floating gate and a select gate, sothis distance may be chosen separately and is not necessarily the sameas the distance between floating gates.

FIG. 10 shows the structure of FIG. 9, along the same cross section,after removal of masking portions 313 a-e. Sidewall spacers 717 b-e andwide dielectric portions 717 a, 717 f remain in place overlying thesecond polysilicon layer 309. Subsequently, sidewall spacers 717 b-e andwide dielectric portions 717 a, 717 f are used as an etch mask topattern underlying layers to form the memory array.

FIG. 11 shows the structure of FIG. 10 along the same cross sectionafter an etch step is carried out to etch through polysilicon strip 305a, second polysilicon layer 309 and second dielectric layer 311,stopping on first dielectric layer 301 or on substrate 303. This etchstep separates second polysilicon layer 309 into separate word lines 309b-e and strips 309 a, 309 f. This etch also separates the strips of thefirst polysilicon layer into separate floating gates 305 m-p. Word lines309 b-e form control gates where they overlie floating gates 305 m-p.Because word lines 309 b-e and floating gates 305 m-p are formed by thesame etch step they are self aligned. Portions 305 l and 305 q are alsoformed under strips 309 a, 309 f. Portion 305 l is electricallyconnected to strip 309 a to form a first select gate. Similarly, portion305 q is electrically connected to strip 309 f to form a second selectgate. Such self aligned structures as those shown in FIG. 11 provideuniform coupling between floating gates and control gates and simplifyfabrication. After the etch step is completed, source/drain regions 11a-e may be formed by implanting dopants into exposed areas of substrate303. These exposed areas lie between floating gates 308 m-p so that thesource/drain regions connect memory cells of a string. Aftersource/drain regions 11 a-e are formed, the memory array may be coveredby a protective layer such as a thick dielectric layer or otherprotective material. Sidewall spacers 717 b-e and wide dielectricportions 717 a, 717 f may be removed prior to forming the protectivelayer or may remain in place when the protective layer is formed.

FIG. 12 shows the NAND array of FIGS. 3-11 in plan view. The crosssections of FIGS. 3 and 4-11 are indicated in FIG. 12 by B-B and C-Crespectively. FIG. 12 shows the NAND array at a stage of formation thatcorresponds to that shown in FIGS. 3 and 4. Photoresist portion 415 c isshown extending across the memory array in the X-direction and also inthe Y direction to form a closed loop. In some memory arrays, severalsimilar concentric loops may be used. The width of photoresist portion415 c forming the closed loop is F, the minimum feature size of thelithographic process used to form photoresist portion 415 c. Betweenphotoresist portions 415 b-d are openings that also have a width of F.An opening 121 a is formed between photoresist portions 415 a and 415 bthat is wider than F. A similar opening 121 b is formed betweenphotoresist portions 415 d and 415 e. It can be seen that photoresistportion 415 e of FIG. 4 is not just a strip extending in theX-direction, but is a strip that extends in both X and Y directions toform a closed, rectangular shaped loop. It can also be seen that thestructure of cross section C-C is formed with a mirror-image structurealso formed using the same photoresist pattern. FIG. 14 includes dottedlines that show the locations of STI structures 307 a-d that underliephotoresist portions 415 a-e, masking layer 313 and second polysiliconlayer 309. STI regions 307 a-d, along with first and second polysiliconlayers 305, 309 are formed prior to forming photoresist portions 415a-e.

FIG. 13 shows the structure of FIG. 12, in the same plan view, afterresist slimming. Thus, the structure of FIG. 13 corresponds to the crosssection shown in FIG. 5. As can be seen, photoresist portions 415 a-ehave become narrower and openings between photoresist portions 415 a-ehave become correspondingly wider as a result of photoresist slimming.In this example, photoresist portions 415 b-d are narrowed to a width ofapproximately F/2 while the openings between photoresist portions 415b-d are increased to a width of approximately 3F/2. Opening 121 abetween photoresist portions 415 a and 415 b has a width ofapproximately 4F. Opening. 121 b between photoresist portions 415 d and415 e also has a width of 4F.

FIG. 14 shows the structure of FIG. 13 after the photoresist pattern ofFIG. 13 is transferred to masking layer 313 as a result of an etchingstep using photoresist portions 415 a-e as an etch mask. Thus, thestructure of FIG. 14 corresponds to the cross section shown in FIG. 6.Third dielectric layer 717 (not shown) is formed over masking portions313 a-e. Third dielectric layer 717 overlies both masking portions 313a-e and exposed areas of underlying second polysilicon layer 309 inopenings between masking portions 313 a-e.

FIG. 15 shows the structure of FIG. 14 after formation of photoresistportions 819 a-d overlying third dielectric layer 717. Thus, thestructure of FIG. 15 corresponds to the cross section shown in FIG. 8.As is shown, photoresist portions 819 a-d are somewhat smaller than theopenings between masking portions. Photoresist portions 819 a-d do nothave to be exactly aligned with openings between masking portions.Subsequent to forming photoresist portions 819 a-d, an etch is carriedout to remove portions of third dielectric layer 717, leaving sidewallspacers 717 b-e and portions third dielectric layer 717 covered byphotoresist portions 819 a-d. Then, photoresist portions 819 a, 819 bare removed and masking portions 313 a-e are removed. Subsequently, anetch is performed to etch the underlying first polysilicon layer 305 andsecond polysilicon layer 309 in the pattern of the remaining portions ofthird dielectric layer 717.

FIG. 16 shows the resulting structure after etching first polysiliconlayer 305 and second polysilicon layer 309. A series of concentric,rectangular shaped, closed loops are formed in the pattern of sidewallspacers 717 b-e. Word lines 309 b-e form portions of these loops. Wordlines 309 b-e have a width of F/2 and are spaced approximately F/2apart. Underlying word lines 309 b-e are STI structures 307 a-d and,between STI structures 307 a-d, floating gates formed from the firstpolysilicon layer 305. Because word lines 309 b-e and floating gates areformed by the same etch step they are self aligned. Word lines 309 b-eform control gates where they overlie floating gates. Word lines areconnected together in pairs in the structure shown, with connectionsformed at either end of word lines by portions of loops that extend inthe Y-direction. Thus, for example, word lines 309 b and 161 b areconnected by portions 163 b and 167 b. Similarly, word lines 309 c-e areconnected with word lines 161 c-e by portions 163 c-e and portions 167c-e. The connecting portions 163 b-e, 167 b-e at word line ends may beformed of portions of both first polysilicon layer 305 and secondpolysilicon layer 309 joined together in a similar way to select lines.Also shown in FIG. 16 are select lines 309 a, 309 f-h. Select lines 309a, 309 f-h have a width that is approximately 4F. A NAND string isformed by a series of floating gate memory cells connected between twoselect lines. Thus, FIG. 16 shows two units 168, 169 of NAND stringsthat have their word lines connected together but have separate selectlines. Source and drain contacts are also shown in FIG. 16. In thepresent example, source contacts 165 a-c are connected together to forma common source contact for all the NAND strings shown. Drain contacts166 a-f are connected to bit lines that run in the Y direction above theword lines.

While the structure of FIG. 16 shows portions 163 b-e, 167 b-e thatextend in the Y-direction to connect word lines 309 b-e and word lines161 b-e together, the space occupied by such portions is generally notsignificant. In particular, a block generally extends much further inthe X-direction than in the Y-direction so that extending a block by asmall amount in the X-direction will not greatly affect the areaoccupied by the block.

FIG. 17 shows a circuit diagram for the structure of FIG. 16. FIG. 17shows three NAND strings connected above a common source line 171 toform the first unit 168, and three NAND strings connected below commonsource line 171 to form the second unit 169. In some cases, first unit168 and second unit 169 are separately erasable, and so may beconsidered as separate blocks. Source select lines. 309 f, 309 g anddrain select lines 309 a, 309 h are connected to circuits that enableportions of the memory array to be separately accessed. However, theseconnections are not shown for clarity in FIG. 17. In addition, wordlines are connected to shared word line decoder and driver circuits 173used to access the memory array. Because word lines of first unit 168and second unit 169 are connected together, shared word line decoder anddriver circuits 173 serve both units. In this way, the amount of spaceon a memory die that is devoted to word line decoder and driver circuitsmay be reduced by half compared with a memory die that does not sharecircuits in this manner. While word lines are connected between adjacentunits 168 and 169, and word line decoder and driver circuits 173 arealso shared between adjacent units 168 and 169, select gate drivercircuits are not shared so that select lines 309 a, 309 f of unit 168and select lines 309 g, 309 h of unit 169 are separately controlled.

Contact Pads

In some cases, forming good connections to word lines may be difficultbecause of the small size of the word lines. For example, where it isdesired to form a plug to connect to a word line in the verticaldirection, the plug will generally have a diameter of F and so extendsbeyond a word line having a width of F/2 and may electrically contact aneighboring word line if there is any misalignment.

In order to provide good contacts to narrow word lines an embodiment ofthe present invention provides contact pads that are formed integrallywith the word lines, the pads having dimensions that are greater thanthe width of the word line (F/2) and may be greater than the minimumfeature size. FIG. 18 shows a pattern of photoresist portions 181 a-caccording to this embodiment including additional openings 183 a-d inphotoresist portions 181 a-c that are used to form contact pads. FIG. 18is similar to FIG. 12 apart from the addition of openings 183 a-d.

FIG. 19 shows the structure of FIG. 18 after resist slimming, transferof the resist pattern to a masking layer and deposition of a thirddielectric layer overlying the masking layer and a second polysiliconlayer. Unlike the prior embodiment, FIG. 19 shows photoresist portions192 a-d overlying additional openings 183 a-d. FIG. 19 also showsphotoresist portions 192 e-h deposited over openings in the innerphotoresist portion 181 a and outer photoresist portion 181 c as before.Photoresist portions 192 a-h may be formed together in a singlepatterning step. Openings and additional openings are protected byphotoresist portions 192 a-h during subsequent etching of the thirddielectric layer. The result is that, when remaining portions of thethird dielectric layer are used to pattern a second polysilicon layer toform word lines, contact pads are formed that are connected to the wordlines. Contact pads have dimensions that are greater than F/2 and mayhave dimensions greater than F.

FIG. 20 shows the structure of FIG. 19 after etching of a thirddielectric layer, removal of photoresist portions 192 a-h and etching offirst and second polysilicon layers. Contact pads 201 a-d are shown withplugs 203 a-d formed in the vertical direction to connect word lines 205a-d, 207 a-d to word line decoder and driver circuits. Plugs 203 a-d mayconnect to conductive lines that are later formed at a higher level overthe memory array. While the space occupied by the contact pads 201 a-dappears significant in FIG. 20, this drawing is not to scale. In realNAND memory arrays, a block extends much farther in the X-direction thanthe Y direction so that an increase of a few times F in the X-directionmay not greatly increase the overall size of a block.

Shield Plate

In some embodiments, shield plates are formed subsequent to formation ofseparate word lines and floating gates. Sidewall spacers and widedielectric portions are generally removed prior to formation of shieldplates. FIG. 21 shows the structure of FIG. 11 along the same crosssection after removal of sidewall spacers 717 b-e and wide dielectricportions 717 a, 717 f and formation of a shield plate 211. A dielectriclayer 213 is first formed that overlies source/drain regions, floatinggates and word lines. Dielectric layer 213 provides electricalinsulation on the surfaces of floating gates and word lines.Subsequently, conductive shield plate 211 is formed. In this example,conductive shield plate 211 is formed of doped polysilicon, though otherconductive materials may also be used. FIG. 22 shows the structure ofFIG. 21 in plan view. Separate conductive shield plates 211, 215 areformed corresponding to unit 168 and unit 169 respectively. Shieldplates 211, 215 are not in electrical contact with floating gates orword lines because dielectric layer 213 separates shield plates 211, 215from floating gates and word lines. However, a shield plate iscapacitively coupled to floating gates so that the voltage of a shieldplate may be used to modify the voltage of a floating gate. Shieldplates 211, 215 allow separate erasing of unit 168 and unit 169 and thusdefine blocks in the memory array. In an alternative embodiment, unit168 and unit 169 may only be erasable together and so form a singleblock.

A shield plate may extend over and between adjacent word lines andfloating gates as shown in FIGS. 21-22 or may extend between adjacentword lines and floating gates but not extend over them as shown in FIG.23. The structure of FIG. 23 may be achieved by applying ChemicalMechanical Polishing (CMP) or an etch-back process to the structureshown in FIG. 21. FIG. 24 shows the structure of FIG. 23 in plan view.Separate conductive portions 233 a-e shown in FIG. 24 may beelectrically connected together so that they form a single conductiveunit. Formation and use of shield plates in NAND memory arrays aredescribed in US Patent Application Publication No. 2005/0180186entitled, “Shield plate for limiting cross coupling between floatinggates.” Shielding in NAND arrays is also described in US PatentApplication Publication No. 2005/0072999, entitled “Bitline directionshielding to avoid cross coupling between adjacent cells for NAND flashmemory.” In the present application, shield plates are not connected tosource/drain regions and are separately controlled so that a voltage maybe applied to a shield plate. Thus, a shield plate generally has aconnection to a shield plate driver circuit that controls the voltage ofthe shield plate.

FIG. 25 shows a flowchart for fabricating a memory array according to anembodiment of the present invention. A first dielectric layer and firstpolysilicon layer are formed on a substrate surface 255 a. Thepolysilicon is generally deposited so that it is doped and thereforeelectrically conductive. In some cases, the first polysilicon layer maybe deposited undoped and later doped. STI structures are formed 255 b bypatterning the substrate, forming trenches that extend into thesubstrate. The trenches are filled with STI material such as Silicondioxide. Subsequently, a second doped polysilicon layer is deposited 255c that overlies both STI structures and first polysilicon portions. Thefirst and second polysilicon portions are separated by a seconddielectric layer. A Silicon Nitride masking layer is formed over thesecond polysilicon layer. Next, a photoresist layer is formed over theSilicon Nitride layer 255 d and is patterned into photoresist portionsthat include one or more concentric, rectangular shaped strips. Aslimming process is performed 255 e on the photoresist portions so thatthe thickness of the strips is reduced to about half the minimum featuresize of the lithographic process used. Then the slimmed photoresistportions are used to pattern the Silicon Nitride layer into portionsthat also have a width that is approximately half the minimum featuresize 255 f. A Silicon dioxide layer is then formed 255 g over both theSilicon Nitride portions and the exposed portions of the secondpolysilicon layer. Photoresist portions are formed 255 h to cover partsof the Silicon dioxide layer that are to be protected. Then ananisotropic etch is performed 255 i that removes Silicon dioxide exceptin locations close to sidewalls of Silicon Nitride portions andlocations covered by photoresist portions. Photoresist portions coverareas that later become source select lines and drain select lines. Insome examples, contact pad areas are also covered by photoresistportions. Photoresist portions are then removed 255 j and SiliconNitride portions are also removed 255 k leaving Silicon dioxide portionson the second polysilicon layer. These Silicon dioxide portions are thenused as a mask to etch the first and second polysilicon layers to formword lines and floating gates 255 l. In some cases, shield plates arethen formed 255 m by depositing a dielectric layer followed by a thirdpolysilicon layer to form a conductive polysilicon plate.

Memory Operation

The operation of a memory array according to an embodiment of thepresent invention that does not use a shield plate will now bedescribed. FIG. 26 shows a portion of a NAND memory array made up offour units, unit A, A+1, A+2, and A+3. Each of the units A through A+3contain many NAND strings with 32 memory cells in each NAND string. UnitA and unit A+1 are similar in structure to units 168, 169 of FIG. 16except that they have 32 word lines instead of 4, and have many NANDstrings. The NAND strings of a unit extend between source select gatesand drain select gates. Thus, unit A comprises many NAND stringsextending between a drain select gate line (SGD Unit A) and a sourceselect gate line (SGS Unit A). In many prior NAND arrays, a blockconsisted of a similar grouping of NAND strings to that of Unit A.However, the term “block” is generally used to describe the minimum unitof erase of a memory array and because unit A is not separatelyerasable, the term “unit” is used instead of the term “block.” However,it will be understood that unit A resembles a block of a prior art NANDarray and has a similar structure. Unit A has 32 word lines, WL0-WL31extending in the X-direction. Only a few word lines (WL0, WLn, WLn+1 andWL31) are shown for clarity, with WLn being a representative word line.As is shown in FIG. 26, word lines WL0-WL31 of unit A are connected toWL0-WL31 of unit A+1. Such connections may be formed as describedearlier in the present application, or in some other manner. Word linesWL0-WL31 may be connected at both ends, forming concentric, closed,rectangular shaped loops. For clarity, connecting portions between unitsare only shown at one end. Adjacent units, such as units A and A+1, areseparately selectable by using select gates, however, because word linesof adjacent units are joined together, such adjacent units are notseparately erasable in the present example. Thus, unit A and unit A+1are erased together and so form a block. Similarly, unit A+2 and unitA+3 are erased together and so form a block. A common source line 261extends between adjacent units A and A+1, and another common source line263 extends between units A+2 and A+3. Bit line connections 265 areformed between unit A+1 and A+2 and also between unit A and an adjacentunit (not shown).

FIG. 27A shows the voltages that are applied to various elements of thememory array of FIG. 26 in order to perform a read operation. A readoperation determines the logical states of memory cells based on theamount of charge stored in floating gates. In this case, the readoperation is performed on cells of word line WLn of unit A+1. Thus, onlyfloating gate memory cells in unit A+1 that underlie word line WLn areread in this operation. Other floating gate memory cells are not read atthis time. The drain and source select gates (SGD and SGS) are set toVSS (0 volts) for all units except unit A+1 so that select transistorsare turned off for all units except unit A+1. Drain and source selectgates for unit A+1 are set to VSG (4.5 volts) to turn on selecttransistors for unit A+1. In this way, bit lines that serve units Athrough A+3 (and other units) are electrically connected only to NANDstrings of unit A+1. Word lines WLn of units A and A+1 have VSS(approximately 0 volts) applied, while all other word lines (WL0 toWLn−1 and WLn+1 to WL31) of units A and A+1 have VREAD (approximately4.5 volts) applied. The result is that floating gate transistors of allword lines except WLn are turned on so that NAND strings are conductiveapart from floating gate transistors underlying WLn. The state offloating gate transistors under WLn may thus be read by passing acurrent through NAND strings and measuring the effect of charge in thefloating gates under WLn. Voltages on word lines of units other thanunits A and A+1 may be allowed to float during reading of unit A+1. Datais read out through bit lines with the common source lines (“Arraysource”) held at VSS (0 volts) and the P-well in the substrate also atVSS.

FIG. 27B shows the voltages that are applied to various elements of thememory array of FIG. 26 in order to perform a program operation. Theprogram operation adds charge to floating gates of memory cells tochange the logical state of the memory cell. In this case, theprogramming operation is performed on memory cells of WLn of unit A+1.This means that the floating gate memory cells underlying WLn, in unitA+1 are programmed, while other cells are not programmed. The drain andsource select gates (SGD and SGS) are set to VSS (0 volts) for all unitsexcept unit A+1 so that select transistors are turned off for all unitsexcept unit A+1. The source select gate for unit A+1 is also set to 0volts. The drain select gate for unit A+1 is set to Vdd (approximately2.5 volts). All word lines except for WLn are set to a voltage of VPASS(approximately 10 volts). WLn receives a voltage of VPGM (approximately20 volts). This relatively high voltage may cause electrons to enter thefloating gate from the substrate depending on a programming voltagesupplied by a bit line. In a common arrangement, programming voltagesare applied by bit lines as a series of pulses. In one example, 0 voltsapplied to a bit line causes programming of a cell. Cells that havereached their desired voltages receive a bit line voltage of Vdd toinhibit further programming. Programming may involve multiple pulsingand verifying steps. Word lines of units other than units A and A+1 areallowed to float during programming of unit A+1. In some examples, bitlines may be used to enable or inhibit programming of individual cellsalong WLn as cells reach their desired state. Cells of a word line maybe programmed together, or in groups.

FIG. 27C shows the voltages that are applied to various elements of thememory array of FIG. 26 during an erase operation. The erase operationremoves charge from floating gates to return them to a base level ofcharge that allows them to be programmed again. Charge is removed bycreating an electrical field between word lines and the substrate thatcauses charge to flow from the floating gate to the substrate. Anappropriate electrical field is created by appropriately biasing wordlines and the underlying P-well in the substrate. Because word lines ofunits A and A+1 are connected together in FIG. 26, the memory cells ofunits A and A+1 are erased together. Drain and source select gates areallowed to float during erase. All word lines WL0-WL31 are set to VSS(approximately 0 volts), while the voltage of the P well in thesubstrate is set to VERA (approximately 20 volts). Thus, a 20 voltdifference exists between a word line above a floating gate and thesubstrate below the floating gate. Floating gates in both units A andA+1 are subject to this electrical field and so they are both erased asa single block. Word lines of other units such as units A+2 and A+3 areallowed to float so no erase occurs in units A+2 and A+3.

FIG. 28 shows an alternative embodiment where shield plates are used toestablish two separately erasable blocks that share the same word lines.FIG. 28 shows blocks B, B+1, B+2 and B+3 connected in a manner similarto units A, A+1, A+2 and A+3 of FIG. 26, but with shield platesprovided. The addition of shield plates allows separate erasing ofblocks because an electric field sufficient to cause charge to beremoved from a floating gate may be created by applying suitablevoltages to the separate shield plates of different blocks.

FIG. 29A shows the voltages that are applied to various elements of thememory array of FIG. 28 during a read operation. These voltages are thesame as those given in FIG. 27A except for the addition of shield platevoltages for each block. The shield plate voltage for block n that isnot being read is VRSP (approximately 4.0 volts). The shield platevoltage for block n+1, which is being read, is also VRSP. While a valueof VRSP in this example is approximately 4.0 volts, in other examples,VRSP may be set at a different voltage, or may be allowed to float. Theshield plate voltages for all other blocks, such as blocks n+2 and n+3may be allowed to float.

FIG. 29B shows the voltages that are applied to various elements of thememory array of FIG. 28 during a program operation. These voltages arethe same as those given in FIG. 27B except for the addition of shieldplate voltages for each block. The shield plate voltage for block B,which is not being programmed, is set at VPSP (approximately 10 volts).The voltage shield plate voltage for block B+1, which is beingprogrammed, is also set at VPSP (approximately 10.0 volts). Other valuesof VPSP may also be used. Shield plates of other blocks, such as blocksB+2 and B+3 may be allowed to float during programming of block B+1.

FIG. 29C shows the voltages that are applied to various elements of thememory of FIG. 28 during an erase operation. These voltages are the sameas those given in FIG. 27C except for the addition of shield platevoltages. Significantly, the addition of shield plates and theapplication of shield plate voltages allows individual blocks to beerased in the present embodiment, where in the embodiment of FIG. 27Cboth units A and A+1 were used together as a single block. A voltage ofVEISP (approximately 18 volts) is applied to the shield plate of blockB, which is not being erased. The application of this high voltageinhibits erase of block B by coupling a high voltage to floating gatesand thereby reducing the voltage difference between floating gates andthe substrate that would cause charge flow. A voltage of VESSP(approximately 5 volts) is applied to the shield plate of block B+1,which is being erased. This relatively low voltage couples to thefloating gates, keeping floating gate voltage relatively low, so a largevoltage difference exists between floating gates and the P-well in thesubstrate (at 20 volts). This large voltage difference causes charge toflow from the floating gate to the substrate. Thus, the addition of ashield plate allows separate erase of different blocks that share thesame word lines.

Although the various aspects of the present invention have beendescribed with respect to exemplary embodiments thereof, it will beunderstood that the present invention is entitled to protection withinthe full scope of the appended claims.

1. A method of forming a nonvolatile memory array on a substrate surfacecomprising: forming a plurality of shallow trench isolation structureson a substrate, individual ones of the plurality of shallow trenchisolation structures extending in a first direction, ones of theplurality of shallow trench isolation structures spaced apart in asecond direction; and subsequently forming a plurality continuousconductive strips, an individual continuous conductive strip includingfirst, second, third and fourth conductive portions, the firstconductive portion extending in the second direction and overlying theplurality of shallow trench isolation structures, the second conductiveportion extending in the second direction and overlying the plurality ofshallow trench isolation structures, the first and second conductivestrips spaced apart in the first direction, the third and fourthconductive portions extending in the first direction to connect thefirst and second conductive portions.
 2. The method of claim 1 whereinthe individual conductive strip forms a closed rectangle, and the first,second, third and fourth conductive portions form the sides of therectangle.
 3. The method of claim 1 wherein the first conductive portionforms a first word line and the second conductive portion forms a secondword line.
 4. The method of claim 1 wherein a width of an individual oneof the plurality of continuous conductive strips is established byspacers formed on sidewalls.
 5. The method of claim 4 wherein the widthis less than a minimum feature size of a lithographic process used toform the nonvolatile memory array.
 6. The method of claim 5 wherein thespacers are slimmed to be less than the minimum feature size of thelithographic process used.
 7. The method of claim 1 further comprisingforming select gate lines that extend in the first direction, the selectgate lines being at least as wide as the minimum feature size.
 8. Themethod of claim 1 further comprising forming floating gates underlyingthe plurality of first conductive portions so that a first plurality ofNAND strings is formed between the plurality of shallow trench isolationstructures.
 9. The method of claim 8 wherein the first plurality of NANDstrings extend between a first select gate line and a second select gateline.
 10. The method of claim 9 further comprising forming a firstconductive shield plate that extends over the first plurality of NANDstrings.
 11. The method of claim 10 further comprising forming floatinggates underlying the plurality of second conductive portions so that asecond plurality of NAND strings is formed between the plurality ofshallow trench isolation structures, the second plurality of NANDstrings extending between a third select gate line and a fourth selectgate line and a second conductive shield plate extending over the secondplurality of NAND strings.
 12. The method of claim 1 further comprisingforming a conductive shield plate that extends between the plurality ofcontinuous conductive strips.
 13. A method of forming a nonvolatilememory array comprising: forming a masking layer that includes aplurality of masking portions extending over a substrate; forming aplurality of sidewall spacers along sidewalls of the plurality ofmasking portions; forming a plurality of word lines extending across thenonvolatile memory array in a first direction and spaced apart in asecond direction, the second direction being perpendicular to the firstdirection, individual word lines having a first width in the seconddirection that is less than a minimum feature size of a lithographicprocess used to form the nonvolatile memory array, the first widthestablished by the width of a sidewall spacer of a plurality of sidewallspacers, an individual word line overlying floating gates of thenonvolatile memory array, individual ones of the plurality of word linesseparated from adjacent ones of the plurality of word lines by adistance that is less than the minimum feature size produced by alithographic process used to form the masking layer; and forming aplurality of select gate lines extending across the nonvolatile memoryarray in the first direction, individual ones of the plurality of selectgate lines having a second width that is at least as great as theminimum feature size, the second width established by the distancebetween two sidewall spacers of the plurality of sidewall spacers. 14.The method of claim 13 wherein individual word lines are linked in pairsby conductive elements extending in the second direction.
 15. The methodof claim 13 further comprising a conductive shield plate that extendsbetween the plurality of word lines.
 16. The method of claim 13 whereinthe masking portions are formed by lithographically patterning aphotoresist layer to form photoresist portions having a width that isequal to a minimum feature size and subsequently slimming thephotoresist portions.
 17. A method of operating a NAND flash memoryarray that has word lines of a first block connected to word lines of asecond block, the first block and the second block sharing a pluralityof bit lines, the first block having a first shield plate and the secondblock having a second shield plate, comprising: erasing data in thefirst block while maintaining data in the second block by applying afirst voltage to the first shield plate while applying a second voltageto the second shield plate.
 18. The method of claim 17 furthercomprising maintaining a common voltage on the word lines of the firstblock and word lines of the second block during the erasing.
 19. Themethod of claim 17 wherein a portion of the substrate underlying thefirst block and the second block is maintained at a third voltage duringerasing, the third voltage being higher than the first or secondvoltages.
 20. The method of claim 19 wherein the first voltage isapproximately 5 volts, the second voltage is approximately 18 volts andthe third voltage is approximately 20 volts.
 21. The method of claim 17further comprising writing data in the first block while not writingdata in the second block by turning on select transistors of the firstblock while turning off select transistors of the second block andapplying programming voltages to the plurality of bit lines.
 22. Themethod of claim 21 further comprising maintaining the first shield plateand the second shield plate at the same voltage during the writing datain the first block.
 23. The method of claim 17 further comprisingreading data from the first block while not reading data from the secondblock by turning on select transistors of the first block while turningoff select transistors of the second block and reading data through theplurality of bit lines.
 24. The method of claim 23 further comprisingmaintaining the first shield plate and the second shield plate at thesame voltage during the reading data from the first block.